The present invention addresses a method for the operation of a test stand for power plants, such as internal combustion engines and the like, the test stand having a brake or power absorption unit (PAU) which absorbs the power of the power plant. A damper is interconnected between the prime mover of the power plant and the PAU.
For the development of power plants, especially internal combustion engines which show a relatively high degree of nonuniformity of torque which causes a variation in angular velocity, devices are necessary for the investigation of the dynamic conditions of the engine such as the dynamic shaft torque. This is generally done in power test stands for power plants whereby the power plant is connected to a cradle mounted PAU by means of a drive connecting shaft. Possible PAUs are electric eddy current dynamometers, water brakes, DC or AC electric dynamometers or hydraulic pumps, which are cradled. The torque of the power plant to be investigated, for instance an internal combustion engine, is measured in the form of force on a given torque arm considering the rotational speed.
Since the PAUs and the internal combustion engines, connected via couplings, represent oscillating systems, the danger of occurrence of torsional resonances exists, especially during the determination of the dynamic shaft torque on the output shaft or the flywheel of the internal combustion engine being investigated. These torsional resonances may lead to the destruction of the interconnecting shaft.
To avoid potentially destructive torsional oscillations, torsionally elastic, dampening coupling shafts are used and arranged between the PAU and the internal combustion engine. However, such torsionally elastic, dampening coupling shafts are sensitive to radial and angular misalignments and are therefore subject to excessive wear if the shafts of the PAU and the internal combustion engine exhibit even a slight such misalignment. Also, the range of operating speeds for which such couplings can be utilized is often not sufficient.
For measuring the dynamic shaft torque, torque sensors are often arranged between the internal combustion engine and the PAU. Such torque sensors must however be oversized because of the torque amplitude increase within the torsional resonance speed range. This leads to a condition where the required accuracy of measurement can no longer be attained.
To avoid oversizing of the torque sensor, an elastic, dampening element may be connected between the motor under test and the torque sensor. Even with such an arrangement the required accuracy of measurement of the dynamic shaft torque cannot be attained, because the mechanical oscillatory energy which is converted into heat in the elastic, dampening element is not measured by the torque meter.
The elastic, dampening element may, however, also be arranged between the torque sensor and the PAU. Because of the additional weight of the elastic, dampening element, however, the critical rotational speed of the PAU with respect to bending vibrations is influenced and decreased to such an extent that the dynamic performance of the internal combustion engine at maximum speed cannot be determined.
In the case of DC machines used as PAUs, torque computers are used to determine torque. The torque reaction of the electric generator is calculated from the magnetic air gap energy. As a result of erroneous input and/or changing losses, the required accuracy is not achieved. In addition there is no provision for simple calibration.
In cases of internal combustion engines with a relatively high degree of non-uniformity of torque, when ignition fails in one cylinder, it is possible that the torque meter may be destroyed. Because of these conditions, torque sensors that have practically no dampening except for material dampening, must be constructed considering the amplitude increase which occurs at the torsional resonance speed. This results, by necessity, in a reduction of measuring accuracy.
Thus in order to determine the dynamic torque of an internal combustion engine as accurately as possible, that is to minimize measuring errors, and in order to avoid destruction of the torque sensor, the torsional resonance speed must be chosen to lie outside of the speed range of the power plant being investigated. Preferably, the torsional resonance speed should lie below the operating speed of the power plant. In this case the system is used above critical speed. The means indicated above for reducing the amplitude increase through incorporation of an elastic, dampening element in the torsional resonance speed range, and arranging the elastic, dampening element in such a way that the dynamic shaft torque will be received directly by the torque sensor is not successful. The reason is because of the decrease in the critical rotational speed with respect to bending for a PAU whose original layout was for maximum motor speed and that no longer can be used at maximum speed because of excessive coupling weight.